metamorphic
Banded iron formation (ironstone), likely hematite–magnetite-rich
Banded iron formation (BIF); chiefly hematite (Fe₂O₃), magnetite (Fe₃O₄), and quartz (SiO₂)
AI-generated identification · may be incorrectDark gray to reddish-brown, finely layered and folded, with metallic to earthy luster; hardness typically 5–6.5, no obvious cleavage, and relatively high specific gravity (about 3.0–5.0). Magnetite-rich portions may attract a magnet.
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Physical properties
Dark gray to reddish-brown, finely layered and folded, with metallic to earthy luster; hardness typically 5–6.5, no obvious cleavage, and relatively high specific gravity (about 3.0–5.0). Magnetite-rich portions may attract a magnet.
Formation & geological history
Chemical sediment deposited in ancient oceans, where dissolved iron precipitated with silica, then compacted and metamorphosed. Most BIF formed during the Precambrian, especially about 2.5–1.8 billion years ago, during Earth’s oxygenation.
Uses & applications
Major global source of iron ore for steelmaking; attractive polished pieces are used as decorative stone and mineral specimens, but it is not usually a gemstone.
Geological facts
The alternating iron-rich and silica-rich bands preserve evidence of changing ocean chemistry and rising atmospheric oxygen. Folding and oxidation can produce the strongly undulating red-black pattern visible here.
Field identification & locations
Identify by dense weight, fine parallel banding, reddish streak from hematite, and possible magnetism from magnetite; common in Australia, Brazil, India, South Africa, Canada, and Ukraine. A streak test and magnet help confirm it.
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